... to Validator.nu. Look for the Profile option.
The only checks that are made for now is for attribute values. Attribute with values that are unquoted are flagged if either the Pedagogical or Polyglot profile is selected. Additionally, attributes without values at all will be flagged if Polyglot is selected. To see this live look for nowrap in the output for google.com.
What the UI should look like, and what checks should be made are all open for discussion. Neither the coding nor the deployment is an issue, what is needed is somebody to step up and to decide what profile(s) they feel are needed, and what checks should be made in that profile. I find such discussions to be time sinks, one that I can ill afford at the present time.
If people want me to continue to contribute via coding, this input needs to be in the form of test cases. HTML vs XHTML is an excellent source for ideas. I intend to support any and all sincere efforts at defining a profile.